Patent Literature 1 discloses a method and device for measuring the diameter and/or degree of eccentricity of the coating layer of a coated optical fiber. With the method and device, a coated optical fiber is irradiated with the luminous flux emitted from a light source along a direction generally perpendicular to the axial direction of the coated optical fiber, and the luminous flux is received by a photosensor that faces the light source and is disposed generally perpendicularly to the axial direction of the coated optical fiber. The diameter and/or degree of eccentricity of a primary coating layer of the coated optical fiber is determined by analyzing an image formed by the photosensor.